1. Field of the Invention
The present invention relates generally to the provision and use of an annealing furnace inner cover which cooperates with a furnace base to define a treatment chamber within which a charge of metal such as steel is housed for annealing, wherein the inner cover carries at least one interior cooling conduit through which cooling water from an exterior source is circulated to expedite the cooling of gas that is circulated within the treatment chamber during cooling portions of an annealing cycle. The present invention is especially well suited for expediting the cooling of stacked coils of steel sheet from an annealing temperature, and preferably utilizes a helix of cooling coils that surround mid-height portions of the stack, wherein the cooling coils are protectively housed within an inwardly facing recess defined by an enlarged diameter mid-height portion of the inner cover both to prevent cooling coil damage due to impact during cover movement, and to ensure that gas circulation within the inner cover is not adversely restricted by the presence of the cooling coils within a gas circulation space that separates the interior of the inner cover from the cylindrical outer surfaces of the stacked coils.
2. Prior Art
Annealing furnaces are well known that employ one or more inner covers to surround a stack or stacks of coils of sheet steel during annealing of the stacked coils. Each stack of coils typically is carried atop a round, upwardly facing base structure having an outer diameter that is at least slightly larger than the largest coil diameter of the stack. Each inner cover typically has an upstanding, generally cylindrical side wall closed at its upper end by a top structure, and has a depending, rim-like bottom lip configured to engage a gas impervious seal that extends circumferentially about the base structure. By this arrangement, the base and the inner cover cooperate to define a treatment chamber wherein the character of gas that is circulated therein during annealing can be controlled to maintain a nonoxidizing atmosphere to prevent deleterious effects such as the formation of oxide scale on the coils of steel sheet.
While many patents disclose annealing furnace features of the type just described, the disclosures of the following patents of Gary L. Coble provide good examples thereof, and their disclosures are accordingly incorporated herein by reference: U.S. Pat. Nos. 4,516,758; 4,611,791; 4,755,236; 5,048,802; 5,562,879; 5,575,970; 5,578,264; and, U.S. Pat. No. 5,681,525. Reference also is made to Gary L.Coble's U.S. Pat. No. 5,756,043 scheduled to issue May 26, 1998. These nine patents are collectively referred to later herein as the "Annealing Furnace Patents."
Annealing furnaces are expensive units which need to have their productivity maximized by being loaded promptly when ready for use, by being put through annealing cycles that are conducted efficiently so as to minimize their length, and by being unloaded promptly when annealing cycles are completed--so that, between occasions when furnace "down time" is required to service, rebuild or replace base components and the like, a maximum tonnage of metal can be annealed. To minimize furnace "down time," movable components such as inner covers need to be durable and capable of occasionally sustaining reasonable impacts when being moved about; and, inner covers should not be provided with interior structures (such as inwardly projecting cooling conduits) that are likely to be damaged due to impact as inner covers are lowered to surround, and are raised from surrounding, charges of metal supported atop annealing furnace bases.
Furnace productivity can be maximized if cycle time can be shortened so that a larger number of annealing cycles that can be carried out in a given period of time. Since a sizable portion of each annealing cycle is consumed by "cooling time" (i.e., a number of hours required for a charge of metal that has been heated to an annealing temperature of typically about 1300 to about 1500 degrees Fahrenheit to cool back to near-ambient temperature), the desirability of diminishing the required cooling time has long been recognized. A brisk flow of cooling gas typically is circulated within the treatment chamber to carry heat energy away from the metal charge, and some heat exchange mechanism may be employed to expedite the withdrawal of heat energy from the circulating gas. The more efficiently that heat energy can be removed from the gas, the more effective the circulating gas will perform in withdrawing heat energy from the metal charge, and the shorter will be the resulting cooling time of the annealing cycle.
Ducting cooling gas outside a treatment chamber to be cooled using a heat exchanger before being returned to the treatment chamber has been proposed, as is disclosed in such U.S. Pat. Nos. 2,479,815 and 3,366,163. Providing cooling devices situated inside inner covers also has been proposed: in U.S. Pat. Nos. 2,439,127 and 2,479,102, for example, cooling units are shown connected to the closed upper end regions of inner covers, with cooling coils being provided inside the covers' upper end regions; and, in U.S. Pat. No. 3,581,810, conductive cooling at opposite ends of coils is presented as another option for diminishing cooling time. None of these proposals have gained broad industry acceptance.
The most widely accepted cooling proposal which currently is in use in industry calls for "in base" forced water cooling of the gas that is circulated within inner covers, as is disclosed in such U.S. Pat. Nos. 3,429,370; 4,287,940; 4,310,302; and 4,445,852. This cooling technique utilizes cooling conduits interposed among massive metal base components that must be capable not only of supporting the weight of a stack of steel sheet coils but also of withstanding the enormous thermal shock and violent "steam lock" shock that results when the cooling conduits (which are "dry" while being heated to the annealing temperature during the heating portion of an annealing cycle) are flooded with cooling water at the initiation of the cooling portion of the annealing cycle.
The shock experienced by massive metal base components, by adjacent refractory members, by other base-associated components such as base-carried cooling fans, and by the cooling conduits themselves often dramatically shortens the service life of annealing furnace bases, and may significantly increase furnace "down time" and operating costs due to the need for frequent base maintenance and replacement of prematurely failed components. Accordingly, even this most widely accepted forced water cooling approach has been implemented in only a fraction of the annealing furnaces in present-day use.
The foregoing and other shortfalls of prior forced water cooling proposals, taken together with the sizable costs associated with providing economic supplies of cooling water and the costs incurred in maintaining coolant supply and return plumbing in an impact-likely environment have caused the use of forced water cooling to be restricted to less than half of the annealing furnaces in present use in North America even though it has been shown that the use of forced water cooling can significantly diminish the lengths of cooling portions of annealing cycles, in some instances by half.